1. Field of the Invention
The present invention generally relates to protective coating systems for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention relates to a coating system that inhibits the formation of deleterious phases in the surface of a superalloy that is prone to coating-induced metallurgical instability.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack and/or may not retain adequate mechanical properties. For this reason, these components are often protected by an environmental coating and/or a thermal barrier coating (TBC) system, the latter of which typically includes an environmentally-protective metallic bond coat and a thermal-insulating ceramic topcoat, referred to as the TBC.
Environmental coatings and TBC bond coats are often formed of an oxidation-resistant aluminum-containing alloy or intermetallic. An example of the former is MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), which is deposited as an overlay coating. An example of the latter includes diffusion coatings, particular diffusion aluminides and platinum-aluminides (PtAl) that contain aluminum intermetallics (e.g., NiAl). Other types of environmental coatings and bond coats that have been proposed include beta-phase nickel aluminide (NiAl) overlay coatings. In contrast to the aforementioned MCrAlX overlay coatings, which are metallic solid solutions containing intermetallic phases, the NiAl beta phase is an intermetallic compound that exists for nickel-aluminum compositions containing about 30 to about 60 atomic percent aluminum. Notable examples of beta-phase NiAl coating materials are disclosed in commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., U.S. Pat. No. 6,153,313 to Rigney et al., U.S. Pat. No. 6,255,001 to Darolia, and U.S. Pat. No. 6,291,084 to Darolia et al. These NiAl compositions, which preferably contain a reactive element (such as zirconium and/or hafnium) and/or other alloying constituents (such as chromium), have been shown to improve the adhesion of a ceramic TBC, thereby increasing the spallation resistance of the TBC. These same compositions can also be used alone as environmental coatings for superalloy components that do not require the thermal protection of a TBC.
The above-noted coating materials contain relatively high amounts of aluminum relative to the superalloys they protect. Conversely, superalloys contain various elements, including refractory elements, that are not present or are present in relatively small amounts in these coatings. When bond coats and environmental coatings of the type described above are deposited on a superalloy substrate, a primary diffusion zone of chemical mixing occurs to some degree between the coating and the superalloy substrate as a result of the concentration gradients of the constituents. At elevated temperatures, further interdiffusion occurs as a result of solid-state diffusion across the substrate/coating interface. The migration of elements across this interface alters the chemical composition and microstructure of both the coating and the substrate in the vicinity of the interface, generally with deleterious results. For example, migration of aluminum out of the coating reduces its oxidation resistance, while the accumulation of aluminum in the substrate beneath the coating can result in the formation of topologically close-packed (TCP) phases that, if present at sufficiently high levels, can drastically reduce the load-carrying capability of the alloy.
Certain high strength superalloys contain significant amounts of refractory elements, such as rhenium, tungsten, tantalum, hafnium, molybdenum, niobium, and zirconium, which are all components of TCP phases. If these elements are present in sufficient amounts or combinations (particularly rhenium at levels exceeding four weight percent and potentially as low as three weight percent), a detrimental type of diffusion zone containing deleterious TCP phases can form following deposition of an aluminum-containing coating. This detrimental diffusion zone, which has been termed a secondary reaction zone (SRZ), is the result of a cellular reaction that can be characterized by a high-angle grain boundary that migrates from the coated surface of the superalloy into its interior in a process that converts the fine Y/Y′ microstructure into a coarse, and much weaker, microstructure of Y and TCP phases in a matrix of Y′. A particularly notable superalloy prone to SRZ is commercially known as MX4, a high-refractory, fourth generation single-crystal superalloy disclosed in commonly-assigned U.S. Pat. No. 5,482,789. Though the MX4 superalloy exhibits superior intrinsic strength relative to earlier-generation single-crystal superalloys, MX4 superalloy articles coated with an aluminum-containing coating are prone to SRZ formation and growth during elevated temperature service, thereby effectively reducing the load-bearing capacity of the alloy as well as likely causing a debit in fatigue life. Coating-induced SRZ formation in other superalloys has been successfully controlled through heat treatments and careful control of pre-coat surface preparation. However, such heat treatments and processing controls have been found to be insufficient in preventing SRZ in the MX4 alloy, presumably due to its very high refractory metal content and particularly its rhenium content (about 4.5 to about 5.75 weight percent). Other alloys with similar refractory metal contents are also believed to be prone to SRZ.
In view of the above, there has been an ongoing effort to develop coating systems that substantially reduce or eliminate the formation of SRZ in high-refractory alloys, as well as provide a suitable bond coat for TBC adherence and a suitable environmental coating for surfaces not coated by a TBC. One result of the development effort has been the formulation of diffusion barrier layers, as disclosed in commonly-assigned U.S. Pat. No. 6,306,524 to Spitsberg et al. Coating alloys taught by Spitsberg et al. contain ruthenium, optionally chromium, with the balance nickel or cobalt, and are deposited so as to be between the substrate and an aluminum-containing coating deposited on the substrate. Commonly-assigned and co-pending U.S. patent application Ser. Nos. 09/681,821 and 09/683,700 to Zhao et al. disclose ruthenium-containing diffusion barrier compositions that are believed to exhibit improved oxidation resistance and resistance to solid-state diffusion of aluminum and other elements between a superalloy substrate and a coating applied to protect the substrate. In addition to ruthenium, the coatings disclosed by the Zhao et al. applications may contain aluminum, chromium, and the base element of the superalloy (nickel, cobalt, or iron). Ser. No. 09/683,800 to Zhao et al. teaches that the disclosed ruthenium-containing barrier coating addresses the problem of SRZ formation in superalloys, including those containing significant additions of refractory elements such as tungsten, rhenium, tantalum, hafnium, molybdenum, niobium, and zirconium.
It would be desirable if improvements in the reduction of SRZ formation and growth could be achieved with superalloys that are especially prone to SRZ as a result of containing high levels of refractory elements, particularly those alloys containing more than three weight percent rhenium, e.g., the MX4 alloy. Such improvements would preferably be achieved with a coating system that can also provide a suitable bond coat for TBC adherence or a suitable environmental coating for surfaces not coated by a TBC.